The European commission is starting an investigation into the pricing of Apple Computer's iTunes digital music service, after consumers complained that downloading tracks was more expensive in the UK than other parts of Europe.

Officials are investigating whether price differentials between the UK and France and Germany of up to 20% are unjust and amount to a breach of EU pricing regulations.

The inquiry comes after Which? - formerly the Consumers Association - wrote to the Office of Fair Trading last September, asking it to look into iTunes' pan-European pricing.

In the UK iTunes charges users around 120 euro cents per track against the 99 cents it charges French and German users.

The OFT later referred the case to the EC, which today said it would begin an investigation.

"The case mainly concerns the fact that prices for the downloading of music tracks from the Apple UK website are, allegedly, substantially higher than the prices for the downloading of the same tracks from the other Apple websites - in particular the French and German websites," an EC statement said.

However, the investigation is also looking into complaints that UK iTunes users are being prevented from accessing cheaper tracks via other European websites.

Music fans in France and Germany must have a bona fide national address and a credit card or other payment method registered in the country to access the iTunes sites.

"The case is also aimed at verifying whether it is true that UK consumers are prevented from downloading the same tracks through the foreign websites and, if so, whether such territorial differentiation is compatible with EU law," the EC added.

Apple declined to comment, but in a response to the original Which? referral said its pricing was based on "the underlying economic model in each country".

"That's not unusual. Look at the price of CDs in the US versus the UK. We believe the real comparison to be made is with the price of other track downloads in the UK," Apple said.

Apple's iTunes has become by far the dominant force in the online music industry since it launched in Europe last June.

Globally it is estimated to have sold over 350m tracks to date, the majority in the US.

Analysts at US securities firm Piper Jaffray predicts iTunes to reach 513m downloads by the end of this year and break through the 1bn barrier in 2006.

But in addition to the EC investigation, Apple faces a number of legal actions, most based on allegations of monopolistic practices.

Earlier this year Californian Thomas Slattery filed a lawsuit seeking unspecified damages against the company for making iTunes' 1m-plus tracks available only to iPod users.

Mr Slattery claims he was "forced" to buy an iPod in order to take his music collection, which is first downloaded onto a computer, with him in a portable format. This, he claims, is a breach of US antitrust laws.

Earlier this month a French group sued both Apple and Sony, alleging that both companies' music services were anti-competitive because they only work with the companies' own music players.